A television receiver may have the capability of selectively displaying picture information in the same color cathode ray tube (CRT) using a deflection current at different horizontal scan frequencies. For example, when displaying the picture information of a television signal according to the broadcasting standard, it may be more economical to utilize a horizontal deflection current at a rate of approximately 16 KHz, referred to as the 1 H rate. Whereas, when displaying the picture information of a high definition television signal or a display monitor data signal, the rate of the horizontal deflection current may be equal to or greater than 32 KHz, referred to as 2nH, where n is equal to or greater than 1.
It is well known to vary the magnitude of the supply voltage of the horizontal deflection circuit output stage, in accordance with the selected horizontal rate of the deflection current, for obtaining similar picture widths using the same horizontal deflection winding.
In a zero voltage switching (ZVS) type of a switch mode power supply (SMPS), the power transistor is turned on when the voltage across its terminals is approximately zero. Advantageously, a ZVS SMPS supplies regulated voltage with a high efficiency or low power dissipation. When a wide range of supply voltages is required, as in the television receiver discussed above, disadvantageously, a ZVS SMPS may not provide the zero voltage switching feature over the entire output voltage range.
In accordance with an inventive feature, a first power supply that includes a first power transistor generates a supply voltage for a deflection circuit output stage, when the deflection circuit output stage operates in a first deflection frequency. A second power supply that includes a second power transistor generates the supply voltage, when the deflection circuit output stage operates in a second deflection frequency. Each of the first and second power supplies selectively generates the supply voltage at a common terminal.
In carrying out a further inventive feature, the first power supply operates as a ZVS SMPS. The ZVS SMPS supplies power when the deflection frequency is at a relatively narrow range of frequencies such as 2nH. Whereas, the second power supply, for example, a series pass regulator power supply, supplies the power when the deflection frequency is at 1 H. Therefore, advantageously, the ZVS SMPS operates in a substantially narrower range of output voltages than without the series pass regulator power supply.
Advantageously, because only one of the first and second power supplies selectively energizes the output stage at any given time, the first and second power transistors can share a common heat sink arrangement. Therefore, advantageously, the dissipated power removal capability of the heat sink arrangement need not exceed the requirement of only that one of the power transistors that dissipates the greater power.
A video display deflection apparatus, embodying an inventive feature, is capable of slelectively operating at a deflection frequency selected from a range of frequencies. It includes a source of a synchronization signal at a frequency indicative of the selected frequency. A deflection circuit output stage is responsive to the synchronization signal for generating a deflection current at the selected deflection frequency. A first power supply including a first power transistor is used for generating a first supply voltage by the operation of the first power transistor when a first deflection frequency is selected. A second power supply including a second power transistor is used for generating a second supply voltage by the operation of the second power transistor when a second deflection frequency is selected. A source of a signal indicative of a frequency of the synchronization signal is used for generating a first control signal that is coupled to one of the first and second power supplies. The first supply voltage is selected, in accordance with the first control signal to energize the deflection circuit output stage, when the first deflection frequency is selected. The second supply voltage is selected, when the second deflection frequency is selected. When the first supply voltage energizes the output stage, the second power transistor is outside a supply current path of the output stage. When the second supply voltage energizes the output stage, the first power transistor is outside the supply current path of the output stage.